If there was a fuel restriction, surely that would apply regardless of whether the car was in gear or not?

I have read that there is a limiter in the fuel pump relay that kicks in at 4000rpm on automatics to stop revving above that point when in park or neutral, The restriction is removed when the car is judged to be in motion.

The restriction was said to be governed by the Hall effect sensor on the speedo, which I don't have. Plus it's a manual.

However, if you're right there is a restriction in the fuel system, where would I start looking? Wouldn't this also cause problems throughout the rev range?